High speed digital private networks have expanded rapidly in the United States over the past several years due to the increased availability and declining cost of domestic common carrier T1 circuits. U.S. based corporate networks require extensions, however, to international locations where digital transmission circuits may be governed by different standards. These extended global networks need to be capable of integrating voice, with it associated signaling information, as well as data over high speed data lines allowing interconnection of computers and private branch exchanges ("PBXs") and other communication devices.
A typical global network today may use a mixture of T1 and fractional T1 digital or pulse code modulation ("PCM") trunks in North America as required by traffic between different network locations. The European portion may use a mixture of CEPT E1 and E1 subrate PCM trunks. This raises problems of interfacing these disparate trunks at their network nodes as well as the problem of interfacing voice signaling signals between terminal telephone equipment designed to operate with different signaling protocols. For example, problems arise when interfacing T1 lines operating at 1.544 Mbps and the European CEPT E1 lines operating at 2.048 Mbps where each system uses channel associated signaling ("CAS").
CAS signaling systems provide for the separation of voice and signaling waveforms. The signaling information is in the form of a two-level on-off signal representing receiver on-hook, receiver off-hook, station alerting "flash" (also referred to as a "wink"), and dial-pulses which encode destination address, i.e., dialed number information. It may be seen that timing of the transitions of the signaling waveforms is critical to the proper interpretation, for example, of a "flash" signal and a dial-pulse address sequence. Both North American and European standards call for similar telephone instrument signaling protocols.
When signaling information is combined with voice for transmission over a packet transmission line, compatibility problems can arise in the prior art due to the manner in which signaling information is packetized and combined with voice for transmission over a packet trunk. Voice data and signaling signals are transported in different packets which may result in some loss of synchronization between them. Although the timing relationship between the voice data and signaling signals is not critical, the timing relationship between successive transitions of the signaling waveforms is critical to their proper interpretation. Nondeterministic delays within the packet network can in the prior art introduce sufficient time-base distortion resulting in incorrect interpretation of the signaling data by the remote receiving equipment unless proper precautions are introduced in the packet system to ensure the signaling data integrity. In particular, nondeterministic delays within the packet network can in the prior art result in incorrect interpretations of dial pulse sequence data.
In addition, several different receiving end signaling protocols must be accommodated. For example, a T1 trunk with a D4 channel bank handles 24 voice channels, each channel sampling voice at 8000 samples per second which is combined with 24 channels of signaling data, with each channel sampled at 1.5 ms intervals. The T1-ESF trunk format specifics a 3 ms sampling interval for signaling data. European CEPT 30 channel trunk format requires a 2 ms signaling sampling interval.